This application is directed to an improved vibration absorber for suspended cables including but not limited to electrical transmission lines; and more specifically to a device for absorbing energy to suppress mechanical subspan oscillation and aeolian vibration of such cables.
The vibration absorber herein described operates on the principles disclosed in co-pending U.S. patent application Ser. No. 147,096, filed May 7, 1980, now U.S. Pat. No. 4,346,255, issued Aug. 24, 1982, assigned to the assignee of the present application and entitled OVERHEAD ELECTRICAL CONDUCTOR SYSTEM INCLUDING SUBSPAN OSCILLATION AND AEOLIAN VIBRATION ABSORBER FOR SINGLE AND BUNDLE CONDUCTORS. This prior application discloses and claims an essentially dissipative (as opposed to spring-type) vibration absorber, having a damping mechanical impedance which essentially matches the mechanical impedance of the transmission line to which the damper is attached. The acceptable range of damper impedance of the absorber is indicated as being anywhere between half and three times the transmission line mechanical impedance.
By essentially matching the transmission line mechanical impedance, and providing a dissipative (i.e., frequency independent) damping effect, the vibration absorber of the aforesaid U.S. patent application Ser. No. 147,096 provides optimum energy coupling between the transmission line and absorber, thus effectively absorbing travelling waves on the line before they build up to large amplitude standing waves which can cause damage to the line and associated supporting elements.
Until the aforesaid invention was made, prior art spring-type dampers had to be designed so that they operated effectively over the resonant frequency range of the transmission line to be damped. These dampers had to also be situated at points on the transmission line where standing waves would be of relatively large amplitude, i.e., at distances of a quarter wavelength from adjacent nodes.
The invention of U.S. patent application Ser. No. 147,096 relates to the use of vibration absorbers which can be connected to transmission lines to provide essentially dissipative damping. That is, these dampers utilize viscous-type effects, so that damping is essentially frequency independent. In contradistinction, those prior art dampers which utilized springs had undesirable resonance characteristics. A typical prior art vibration damper of this type is shown in U.S. Pat. No. 3,885,086. The vibration damper shown in this patent, however, is unsuitable for use in the arrangement contemplated by the aforementioned U.S. patent application Ser. No. 147,096, because it is incapable of providing the critical dissipative damping required. In U.S. Pat. No. 3,885,086, the annular washers 12 are situated between clamp arms 16 and adjacent frame portions 10, and secured thereto so that said washers do not rotate. The washers are of a resilient material, so that rotation of the clamp arm 16 results in deformation of the washers, the resilient characteristics of which then return the clamp arms to their initial orientations. In this arrangement, the only dissipative damping effects are provided by hysteresis losses within the resilient washers. Any attempt to increase the hysteresis losses by increasing the size of the washers, results in the spring force of the washers rising substantially faster than their hysteresis losses, making such a design impractical. Further, limitations of the resilient material itself make it impracticable to obtain sufficiently great hysteresis losses to provide critical dissipative damping. In addition, the hysteresis losses in the washers 12 are dependent upon both frequency and amplitude of vibration.
In contradistinction, dampers of U.S. patent application Ser. No. 147,096, being dissipative and therefore frequency-independent, need not be concerned with the resonant frequencies of the transmission line to which they are to be attached. Further, such dampers, being essentially impedance matched (i.e., within a range of half to three times the characteristic impedance of the transmission line to which they are to be attached), absorb travelling waves, so that they can be placed at any desired place on the transmission line to be damped.
Thus, there remains a need for an improved vibration absorber capable of being utilized according to the aforementioned principles.
Accordingly, an object of the present invention is to provide an improved vibration absorber in which the damping effect produced is essentially dissipative and therefore frequency-independent, and wherein the damping impedance is adjustable to a value capable of essentially matching the mechanical characteristic impedance of the cable to which the vibration absorber is connected.